This invention relates to the wet grinding or coal or ores containing metal values with a grinding aid.
The wet grinding of mineral ores is employed as an intermediate process in the extraction of metals from their ores. The process is one which increases the surface area of the material and also, by reduction of the average particle size of the mineral, facilitates subsequent process steps, e.g., flotation separation or chemical treatment. Grinding operations for ores or coal are usually carried out in mills such as ball, bead, rod or pebble mills, depending upon the degree of comminution required. Autogenous grinding is also employed.
In the processing of ores, the essential step is the comminution of the ore down to the size at which valuable metal grains are released from the gangue matrix. With the inevitable trend towards working of lower-grade ore deposits, the amounts of metals liberated tend to decrease and the grinding cost per ton of product increases. This factor along constitutes a considerable fraction of the overall cost of winning and the increase in cost of energy has made grinding costs a very significant factor in operations for recovering valuable metals.
The amount of breakage per unit of time (breakage kinetics) and mass transfer of grinding mineral ores are usually controlled by the addition and removal of water, an excellent medium because of its high polarity, to the mill. When the mass transport of the slurry through the mill decreases, the mill operator takes corrective action by either decreasing the solids feed rate and/or temporarily increasing the amount of water entering the mill. While both actions will prevent the mill from overloading, mill efficiency is reduced because fewer solids are ground per unit of time under such conditions.
Additionally, it is well known that the traditional tumbling mill apparatus used for wet-grinding ores are extremely inefficient in energy utilization, wasting (based on theoretical bond breakage energies) perhaps as much as 90% or more of the energy supplied to the mill. Therefore, even small increases of a few percent in the reduction of size distribution of ore particles and an increase in throughput of ore ground per unit of time would significantly improve the efficiency of grinding and cost of mill operations, especially with respect to energy utilization.
While various methods and chemical agents that act as grinding aids have been employed in efforts to increase grinding efficiencies and economics, these efforts have at least been only partially beneficial and many have even proved to be contradictory in related downstream processing operations. Various chemical agents, e.g., dispersants, surfactants, polysiloxane, organosilicones, glycols, amines, graphite, non-polar liquids and the like have all been utilized and may increase the rate of grinding by preventing particle agglomeration. However, as reported in Perry's Chemical Engineering Handbook, 5th Ed. 1973, at Sec. 8-12, there really is no scientific method of choosing the most effective surfactant. Rather, surfactant lists and kits that can be used for systematic trails are made available.
Certain polyelectrolytes have also been employed in grinding operations. For example, polyacrylic acid materials have been used in the dry grinding of Bentonite (U.S. Pat. No. 3,220,946) and as dispersing agents in the wet grinding of chalk, fillers and pigments (U.S. Pat. Nos. 3,604,634 and 3,934,825 are representative) for particle size reduction. However, none of these operations concern the grinding of coal or mineral ores for liberation of metal values. U.S. Pat. No. 3,950,182 teaches the grinding of coal, fillers and metal ores, the operations being carried out with ionic polysaccharides, such as sodium carboxymethyl cellulose. U.S. Pat. No. 3,252,662 concerns the grinding of metal ores with a deflocculating agent comprising linear condensed phosphates, preferably sodium tripolyphosphate.